From: Jun Yang tipc_node_link_down() caches the link pointer before taking n->lock: struct tipc_link *l = le->link; /* unlocked */ if (!l) return; tipc_node_write_lock(n); if (!tipc_link_is_establishing(l)) { /* deref l */ ... tipc_link_reset(l); /* write into l */ if (delete) { kfree(l); le->link = NULL; The delete=true caller frees that very object under n->lock, so the lock does not protect the cached pointer against it: - CPU A, delete=false: tipc_rcv() on TIPC_LINK_DOWN_EVT, or the link supervision timer via tipc_node_timeout(), reads l unlocked and then dereferences it under n->lock; - CPU B, delete=true: netlink TIPC_NL_BEARER_DISABLE -> bearer_disable() -> tipc_node_delete_links() -> tipc_node_link_down(n, bearer_id, true) -> kfree(l). The link is freed with plain kfree(), not kfree_rcu(), and for UDP bearers disable_media() only schedules the asynchronous cleanup_bearer() work, so its synchronize_net() runs after the links are already gone. An in-flight CPU A that has read l therefore dereferences freed memory once B frees it: a use-after-free read in tipc_link_is_establishing(), and a use-after-free write via tipc_link_reset() on the establishing branch. BUG: KASAN: slab-use-after-free in tipc_link_is_establishing+0x3f/0x50 Read of size 4 at addr ffff888111f90868 by task swapper/3/0 tipc_link_is_establishing+0x3f/0x50 tipc_node_link_down+0x1a0/0x450 tipc_node_timeout+0x63d/0xca0 Allocated by task 7903: tipc_link_create+0x17e/0xf20 tipc_node_check_dest+0x98b/0x1150 tipc_disc_rcv+0x9f3/0x10c0 tipc_udp_recv+0x42a/0x7e0 Freed by task 7903: tipc_node_link_down+0x301/0x450 tipc_node_delete_links+0xf5/0x240 bearer_disable+0xc0/0x240 __tipc_nl_bearer_disable+0x21f/0x380 Move the le->link read inside tipc_node_write_lock(), so it is serialised against the kfree() in the delete path. A racing teardown now either has not run yet, and we see a valid link, or has already run, and we see NULL. Fixes: 73f646cec354 ("tipc: delay ESTABLISH state event when link is established") Cc: stable@kernel.org Reported-by: TencentOS Corvus AI Signed-off-by: Jun Yang --- net/tipc/node.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-) diff --git a/net/tipc/node.c b/net/tipc/node.c index 8e4ef2630ae4..918e59362d07 100644 --- a/net/tipc/node.c +++ b/net/tipc/node.c @@ -1063,16 +1063,23 @@ static void tipc_node_link_down(struct tipc_node *n, int bearer_id, bool delete) { struct tipc_link_entry *le = &n->links[bearer_id]; struct tipc_media_addr *maddr = NULL; - struct tipc_link *l = le->link; int old_bearer_id = bearer_id; struct sk_buff_head xmitq; - - if (!l) - return; + struct tipc_link *l; __skb_queue_head_init(&xmitq); + /* Read le->link under n->lock: the delete path below kfree()s it while + * holding the same lock, so an unlocked read here could be raced by a + * concurrent bearer teardown and leave us with a freed pointer. + */ tipc_node_write_lock(n); + l = le->link; + if (!l) { + tipc_node_write_unlock(n); + return; + } + if (!tipc_link_is_establishing(l)) { __tipc_node_link_down(n, &bearer_id, &xmitq, &maddr); } else { -- 2.55.0